The goal of this research proposal is to elucidate the signaling of the two isoforms of the D2 dopamine receptor. These receptors are alternative splice forms of the same gene, which differ at the protein level by the addition of 29 amino acids in the third cytoplasmic loop, generating a long and short form. The two isoforms are co-expressed in all tissues, albeit in varying ratios. Previous work has shown that these two receptors couple to distinct Gi proteins to regulate cellular effectors, such as adenylyl cyclase. This proposal will focus on elucidating the regions of each isoform which are responsible for its specific interaction with G proteins. The approach proposed is the creation of point mutations in both isoforms and expression of these mutants in a Gi-specific background to assess functionality. Using this approach will allow the definition of amino acids which dictate the specificity of receptor-G protein interaction. Ultimately, the goal is to elucidate which amino acids residues of these isoforms are responsible for the specificity at the receptor-G protein interface. The specific aims of this proposal are to: 1) create identical point mutations in both D2s and D2l 2) express the mutant receptors in a Gi specific background 3) assess the effect of mutation on signaling to adenylyl cyclase 4) assess the effect of mutation on the physical interaction with G proteins. The D2 dopamine receptors are implicated in several diseases, such as Parkinson's and schizophrenia; understanding the mechanisms that govern the specificity of the signaling of the D2 dopamine receptors may have important implications in disorders thought to result from dopaminergic dysfunction.